Catherine Walker closed her eyes, pressed her hands over her ears, and tried to escape.

It’s been four months since Los Angeles police killed her son, Grechario Mack, whose death barely made headlines, who did not become a viral hashtag. On a recent afternoon, the 59-year-old mother wore pins with her son’s face and said she was ready to speak. But when the moment came, she could hardly talk.

As relatives recounted the killing around her, she tried to shut out the words describing Mack’s last moments. Eventually, she collapsed in her chair in anguish.

“I couldn’t save my baby,” she cried as someone held her. “When they took my son’s life, they took a part of me.”

Police shot Mack, a 30-year-old father of two, in the middle of a mall on the afternoon of 10 April, as he was holding a kitchen knife and having a mental health crisis. Less than 24 hours later, officers arrived at a park and killed Kenneth Ross Jr, another black resident who struggled with mental illness and was said to be fleeing when police shot him with a military-style rifle.

The two families, brought together by Black Lives Matter the day of Ross’s death, are now channeling their grief into a fight for justice – taking on one of the country’s deadliest police systems, where law enforcement killings of black mentally ill residents are so normalized, families struggle to be heard. They face an uphill battle in the most secretive state in the US for police misconduct, in a region where officers who shoot are never prosecuted.

“Mentally, I can’t even do nothing right now,” said Fouzia Almarou, Ross’s mother. “But I’m gonna stay strong … I want to make sure my son is remembered.”

From 2010 to 2014, police in LA county shot 375 people, about one person every five days. Black residents make up 9% of the population, but represented 24% of deaths.

Across the US, the odds are stacked against families who look to courts for justice. Charges are extremely rare and convictions even rarer, with the law widely protecting officers who claim they feared for their lives. In LA, the odds of prosecution are effectively zero.

Since 2000, there have been no charges for the more than 1,500 shootings by police in the county. Since the district attorney Jackie Lacey was elected in 2012, roughly 400 people have been killed by on-duty officers or died in custody, according to Black Lives Matter LA. Lacey even declined to file charges when the chief of the LA police department (LAPD) called for the prosecution of one of his own officers.

“It really greenlights this type of behavior,” said Melina Abdullah, a BLM organizer in LA. “Police don’t have to care about anybody’s life, especially if they’re black or brown or poor.”

Abdullah and other activists are part of the Justice Teams Network, which provides “rapid response” after killings. They go to the scenes, interview witnesses, offer the family assistance with press and funerals, and work to counter the police narratives.

On a recent afternoon, Abdullah took the Guardian to sites of police killings in south LA. One stop was a quiet alley where three years earlier, LAPD officers had killed Redel Jones, a 30-year-old woman who had a kitchen knife and was fleeing police.

Jones, who had struggled on and off with homelessness, loved web design, dancing, cartoon shows, electronic music and rap and had a “brain that was always moving”, said Marcus Vaughn, Jones’s husband, recounting their dream of traveling in a mobile home together.

Headlines, however, reduced her to a “suspect” wanted for a robbery. And two years later, Lacey, the prosecutor, reduced her case to a statistic, clearing the policeman with her standard finding of “lawful self-defense”. The district attorney’s office declined an interview request.

“They did not care about Redel. Her death was one less black person. How are you just gonna kill a woman like she just meant nothing?” said Vaughn, adding that Jones was less than five feet tall and had bipolar disorder and depression, but was not violent. “If she was a short little white woman, they would’ve treated her with so much tenderness.”

Abdullah said she felt an obligation to organize after each killing and a sense of relief when a day passed without a death. Standing near the site of Jones’s killing, she was pained to see a makeshift altar had disappeared and vowed to rebuild it.

Jones didn’t get justice, Abdullah said, but she is hoping her next cases could be different.

‘Your aim was to murder my child’

When Quintus Moore saw a TV report saying LAPD officers had shot someone inside the Baldwin Hills Crenshaw mall, he said he felt sad a man had died for no good reason. Later, it dawned on him that he hadn’t heard from his son since the day before.

After a series of frantic messages to each other, a visit to the mall and a call with the morgue, the family discovered that their worst fears were true: Grechario Mack was the victim.

It was supposed to be a celebratory month for Mack. He had been released from prison on 5 April, five days before the killing, and the family had gathered for a “welcome home” party. Mack had had mental health struggles and past run-ins with the law, and, according to his parents, he was on new medication that was negatively affecting him.

Moore said his son had seemed agitated the morning of his death, and that he might have been paranoid or anxious and holding the knife to feel safe.

The LAPD’s report said Mack appeared to be having a “mental health crisis” and was “aggressively waving a long knife”. Police alleged he ignored commands and “ran in the direction” of patrons, leading to the shooting. Two officers fired at him, according to one report.

Abdullah, the BLM organizer, rushed to the mall, located in a black neighborhood and just a few blocks from Redel Jones’s killing. She said mall employees told her that Mack had been talking to himself and seemed unwell, but was not attacking anyone.

One employee of a nearby store, who declined to give her name, told the Guardian she walked within 10ft of Mack, who did not scare her: “He was just standing there … It wasn’t such a big knife.”

Blurry videos from witnesses captured heavily armed officers surrounding Mack and firing a handful of loud shots. Screams echoed throughout the mall as shoppers ducked for cover and ran. When investigators arrived, he was surrounded by shattered glass.

The county’s autopsy said Mack suffered at least five gunshot wounds, including one in his back just below his head.

“It’s like they got some kind of mandate to kill our black young men,” said Moore, who wears his son’s ashes in a pendant around his neck.

Mack’s mother compared the killing to a lynching: “They only went from the noose to the gun … Who gives them the right to be the executioner and the judge?”

Abdullah helped Mack’s family organize a vigil. There, she met Fouzia Almarou, who had more bad news: police had just shot and killed her son, Kenneth Ross, in a park 10 miles south of the mall, one day after Mack’s killing.

Police have provided few details about the killing in the LA suburb of Gardena. Lt Steve Prendergast told the Guardian that officers were responding to calls of shots fired and ended up chasing Ross, 25, whom they considered a suspect and was “running away from the scene”.

Prendergast said there was a “gun found at the scene”, but he couldn’t say whether Ross owned it or had pointed it. One police report said Ross briefly hid in a bathroom and that police shot him with an AR-15 rifle after he exited. That report said the gun had been in his pocket.

The county’s official autopsy said he was shot multiple times, including in the back.

Almarou said her son, who leaves behind seven younger siblings and a four-year-old son, had bipolar disorder and schizophreniabut was well known to local residents as harmless.

“Why did they shoot him in the back?” she said. “Your aim was to murder my child.”

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At the vigil, Almarou ended up finding somecomfort from Mack’s family, who later donated money to Ross’s funeral.

“It allows the most abusive officers to continue to operate,” said George Galvis, executive director of Communities United for Restorative Youth Justice, which co-sponsored legislation to increase transparency. Another bill would stipulate that police could only use deadly force when “necessary”, instead of the current “reasonable” standard. The move, he said, would encourage police to treat people of color the way they often respond to white suspects – de-escalate the situation and work to keep them alive.

LAPD has adopted policies meant to encourage police to defuse tense situations, but critics say the reforms aren’t working and aren’t enough.

Fouzia Almarou, who son, Kenneth Ross, was shot by police. Photograph: Dan Tuffs for the Guardian

Mack, known as Chario, was an honor roll student who graduated high school early, his mother said. He loved to fish and was fiercely protective of family. His nine-year-old daughter wrote a tribute saying she would miss piggyback rides and museum trips, adding: “I know that you’re always in my heart.”

Arianna Moore, Mack’s sister, said her brother motivated her to be courageous: “He would tell me, ‘You could do anything you put your mind to.’”

Vaughn, Redel Jones’s husband, said he and their children sometimes struggled to remember what her voice sounded like. His nine-year-old daughter often wakes in the middle of the night shaking after a nightmare watching her mother die. She fears the police.

Ross, an avid skateboarder, was so generous, his mother recalled, that as a child he gave his allowance money to homeless people: “His heart was amazing.”

Ross’s mother said she was a survivor of domestic violence and that her son took care of her.

When times were tough, she said, her son offered the same message of comfort: “You’ll always have me to take care of you.”